1. Technical Field
The present invention relates to a temperature-compensated oscillator allowing reduction of power consumption while keeping the accurate temperature compensation, and an electronic device equipped with the temperature-compensated oscillator.
2. Related Art
In the past, a crystal oscillator such as a temperature-compensated crystal oscillator (TCXO), which is less susceptible to ambient temperature and inherent characteristics of electrical components, and is superior as a stable oscillation circuit, has been used as a reference clock source of an electronic device such as a microcomputer or a cellular phone.
FIG. 9 shows a temperature-compensated crystal oscillator described in JP-A-2003-258551 (Patent Document 1). As shown in FIG. 9, the temperature-compensated crystal oscillator 100 is composed of an oscillation circuit 102 and a temperature compensation circuit 106. The oscillation circuit 102 has a structure in which a plurality of series circuits each composed of a switch Sn (n denotes a natural number) and a capacitor Cn (n denotes a natural number) is connected to a circuit including a crystal vibrator 104 as an oscillation source, and by setting the switches Sn to ON/OFF, it is possible to vary the capacitance inside the oscillation circuit 102 to thereby control the oscillation frequency of the oscillation signal. On the other hand, the temperature compensation circuit 106 selects a correction value for controlling the frequency so as to reduce the variation in the oscillation frequency of the crystal vibrator 104 due to the temperature variation based on the temperature information obtained by a temperature sensor 108, and then outputs signals for switch control according to the correction value to the oscillation circuit 102. Further, it results that in the oscillation circuit 102, the switches S1, . . . , Sn are individually switched ON/OFF in accordance with the signals for switch control input thereto.
In the temperature-compensated crystal oscillator described in JP-A-62-38605 (Patent Document 2), although being composed of the oscillation circuit and the temperature compensation circuit, the oscillation circuit is provided with a varactor diode with a capacity varying in accordance with the voltage applied thereto, and the temperature compensation circuit outputs a control signal for controlling the capacitance value of the varactor diode so as to reduce the frequency variation of the crystal vibrator due to the temperature variation to thereby vary the frequency. Thus, the oscillation circuit applies the voltage corresponding to the control signal to the varactor diode.
Therefore, in the temperature-compensated crystal oscillator in Patent Document 1 or 2, it results that the frequency variation (an approximate value thereof in Patent Document 1) due to the capacitance inside the oscillation circuit has opposite temperature characteristics to the temperature characteristics of the deviation of the oscillation frequency of the crystal vibrator.
Therefore, the temperature-compensated crystal oscillator of Patent Document 1 or 2 is capable of reducing the variation in the temperature characteristics of the oscillation frequency of the crystal vibrator by the frequency variation due to the capacitance variation inside the oscillation circuit to thereby output the oscillation signal having temperature characteristics of low temperature dependency, a similar technology to which is also disclosed in JP-A-2007-208584 (Patent Document 3).
However, in the temperature-compensated crystal oscillator 100 of Patent Document 1, there are a problem that the frequency changes rapidly due to the change in the capacitance since the change in the capacitance is discrete, and a problem that the cost is too high since it is required to increase the number of capacitors Cn in order to improve the accuracy of the temperature compensation.
Further, the temperature-compensated crystal oscillator of Patent Document 2 has the configuration of driving the temperature compensation circuit if the frequency of the oscillation signal runs off a certain acceptable range centered on a reference frequency. However, since the frequency of the crystal oscillator is controlled using digital data, there is a problem that the frequency changes rapidly similarly to the case of Patent Document 1 on the ground, for example, that there exists a difference between the value to be compensated when resuming the temperature compensation and the compensation value by the digital data. Further, it is necessary to always drive the temperature compensation circuit without setting the acceptable range in order to perform more accurate temperature compensation, and on this occasion, there is a problem that the power consumption of the temperature compensation circuit increases.